The present invention relates to implantable medical devices. More particularly, the invention relates to implantable medical devices that can measure and store parametric data pertaining to operating characteristics of the device, so that later, when a medical practitioner performs a follow-up examination of a patient, the stored parametric data can be retrieved to determine if the device must be repaired or replaced.
Implantable medical devices have become widely used for the diagnosis and treatment of a variety of medical conditions. Some implantable medical devices (known as "implantable cardiac stimulating devices") are designed to monitor and stimulate cardiac tissue of patients who suffer from cardiac arrhythmias. One type of implantable cardiac stimulating device (known as a "pacemaker") is commonly used to treat bradycardia--a condition in which the patient cannot maintain a physiologically acceptable heart rhythm. These devices deliver low energy electrical pulses to the cardiac tissue to establish a regular heart rhythm at a physiologically acceptable rate.
The earliest pacemakers operated asynchronously at a fixed rate (i.e., without regard to physiological need). Thus, if the patient's intrinsic heart rate happened to be about normal for a period of time, the fixed-rate pacemaker would continue to deliver pacing pulses, thereby wasting limited energy reserves. Also, fixed-rate pacemakers were unable to adjust the patient's heart rate in accordance with increased (or decreased) physical exertion. This had the effect of limiting the kinds of activities that a bradycardia patient could engage in, because the fixed-rate pacemaker could not elevate the patient's heart rate to a level that would meet the increased metabolic demands resulting from physical activity. Also, fixed-rate pacemakers occasionally caused discomfort during sleep and rest because, while most people experience a decreased heart rate during these periods, the heart rates of bradycardia patients were held at the fixed rate.
Subsequent advances in pacemaker technology addressed the above-mentioned difficulties related to fixed-rate pacemakers. For example, demand pacemakers include a sensing function to sense the patient's natural cardiac rhythm. When the patient's natural rhythm falls below a predetermined rate, the demand pacemaker delivers pacing pulses. Otherwise, the demand pacemaker inhibits delivery of the pacing pulses, thereby conserving energy. Sensing is typically accomplished by monitoring the patient's intracardiac electrogram (IEGM), and in particular, by counting the number of R-waves which appear in the IEGM over time. However, recent advances have included the ability to directly measure the mechanical activity of the patient's cardiac tissue, as described in the copending, commonly-assigned U.S. patent application Ser. No. 08/091,636, filed Jul. 14, 1993, entitled "Implantable Leads Incorporating Cardiac Wall Motion Sensors and Method of Fabrication and a System and Method for Detecting Cardiac Arrhythmias Using a Cardiac Wall Motion Sensor Signal."
Rate-responsive pacemakers represent another major advance over the above-described fixed-rate pacemakers. Rate-responsive pacemakers include the ability to monitor an indicator of physical activity, and to vary the pacing rate in accordance with the measured level of activity. A variety of indicators have been used to determine whether, and to what extent, the patient is engaged in physical activity. For example, U.S. Pat. No. 4,712,555 (Thornander et al.), which is hereby incorporated by reference in its entirety, describes a rate-responsive pacemaker that uses a physiologic sensor that measures the depolarization time interval between an atrial stimulation pulse and the responsive atrial or ventricular depolarization. The measured time interval serves as an indicator of physiologic need. The time interval between a ventricular stimulation pulse and the resultant ventricular depolarization may also be used as a physiologic indicator. As another illustration, U.S. Pat. No. 4,940,052 (Mann et al.), which is also incorporated by reference in its entirety, describes a rate-responsive pacemaker that uses a piezoelectric transducer as a physiologic sensor. Accelerometers have also been proposed as physiologic sensors, as described in the copending, commonly-assigned U.S. patent applications Ser. No. 08/059,698, filed May 10, 1993, entitled "Miniature Hybrid-Mountable Accelerometer-Based Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication" and Ser. No. 08/091,850, filed Jul. 14, 1993, entitled "Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based Sensor and Method of Fabrication."
Other types of implantable cardiac stimulating devices have become increasingly important for the treatment of cardiac arrhythmias other than bradycardia. For example, implantable cardioverters and defibrillators are used to treat patients who are susceptible to recurrent episodes of ventricular tachycardia or fibrillation. Ventricular tachycardia is characterized by an abnormally high heart rate, perhaps up to 200 beats per minute. To terminate this type of arrhythmia, a cardioverter may deliver a cardioversion shock to the cardiac tissue. Cardioversion shocks typically have a much higher energy content than pacing pulses--on the order of about 2 joules to about 5 joules. But it should also be noted that modern pacemakers can be programmed to deliver pacing pulses in a sequence that is known to interrupt tachycardia, such devices being referred to as "antitachycardia pacemakers."
Fibrillation is the most severe cardiac arrythmia. Although often categorized as a type of tachycardia, it is different in the sense that it is difficult, and sometimes impossible, to distinguish individual heartbeats during fibrillation--whereas in classic tachycardia, each R-wave is typically discernable. When the patient's heart fibrillates, it quivers chaotically, and it thereby does not properly fill with and subsequently eject blood. To terminate this arrhythmia, a defibrillator may deliver a high energy shock to the cardiac tissue--on the order of about 10 joules to about 40 joules.
Implantable cardiac stimulating devices which deliver multiple forms of therapy are also known. Such devices are useful for bradycardia patients who are also susceptible to episodes of ventricular tachycardia or fibrillation. These devices may provide "tiered therapy," in which the type of therapy applied (e.g., bradycardia pacing pulses, antitachycardia pacing pulses, cardioversion shocks or defibrillation shocks) is determined in accordance with the type of cardiac arrhythmia detected--with more aggressive therapies being applied in response to more severe arrhythmias. And if a less aggressive therapy fails to interrupt an arrhythmia episode after a predetermined period of time or number of attempts, tiered therapy devices can heighten the level of therapy applied.
Despite the functional differences among the above-described types of implantable cardiac stimulating devices, they are similar in several respects. For example, the typical device includes pulse generating circuitry and a power supply (i.e., a battery) disposed within a bio-compatible housing. The device is usually implanted beneath the skin on the patient's chest, although other suitable locations may be selected.
Stimulation pulses are usually delivered to the cardiac tissue through at least one stimulation electrode disposed within a lead that is connected between the implantable cardiac stimulating device and the cardiac tissue. Catheter-type leads which are transvenously guided from the device to the cardiac tissue are most frequently used; however, other types of leads, such as epicardial patches, may also be used.
After a cardiac stimulating device is implanted in the patient, a medical practitioner will typically perform periodic follow-up examinations to evaluate the performance of the device. This evaluation is typically accomplished through the use of an external programmer/analyzer which has the ability to telemetrically communicate with the implantable cardiac stimulating device. Using the programmer/analyzer, the medical practitioner can send instructions to the implantable cardiac stimulating device which cause the device to operate in a different way. For example, the medical practitioner can telemetrically increase the resting heart rate maintained by a pacemaker if the patient has experienced episodes of lightheadedness during rest.
The medical practitioner can also use the programmer/analyzer to transmit commands which cause the implantable cardiac stimulating device to take parametric data measurements. As used herein, the term "parametric data" refers to information pertaining to the operating characteristics of the implantable cardiac stimulating device. By causing the device to take such measurements and then evaluating the results, the medical practitioner can determine whether the device is functioning properly, or whether repair or replacement is necessary.
Lead integrity is an operating characteristic that is commonly evaluated during follow-up examinations. Since the leads are implanted, they are subject to stresses caused by patient mobility, bodily fluids, and the like, and as a result, they may deteriorate. Indeed, one condition commonly encountered in pacemaker patients is known as "twiddler's syndrome." These patients absentmindedly manipulate the device implanted beneath the skin, thereby occasionally causing lead damage as the leads twist and turn with the device.
To evaluate lead integrity, the medical practitioner generally directs the implantable cardiac stimulating device to measure the lead impedance. This parametric data measurement indicates whether the lead connection between the implantable cardiac stimulating device and the patient's cardiac tissue remains uncompromised. Lead impedances of about 400 .OMEGA. to about 750 .OMEGA. are typical.
An excessively low lead impedance measurement suggests that the insulation on the lead has deteriorated, causing an undesirable short circuit to the surrounding tissue (other than the targeted cardiac tissue). A short circuit may cause the stimulation pulses to be discharged to the surrounding tissue instead of the cardiac tissue, thereby rendering the stimulation pulses less effective, or perhaps ineffective. In addition, a short circuit may lead to a rapid depletion of energy reserves from the battery.
A high lead impedance measurement indicates that the lead may have fractured. This open circuit condition also prevents the stimulation pulses from reaching their intended destination. Thus, if the lead impedance falls outside the expected range, a surgical procedure to repair or replace the lead may be warranted. Lead impedance may be derived from two other parametric data measurements taken by the device--pulse voltage and pulse current.
Another operating characteristic that is closely monitored during follow-up visits is battery life. Although most implantable cardiac stimulating devices are assigned recommended replacement times by the manufacturer, there may be situations under which the battery drains at an unexpectedly high rate. To determine remaining battery life, the medical practitioner can telemetrically direct the device to measure the internal impedance of the battery (internal battery impedance is known to increase as the battery depletes). When the battery is nearing the end of its useful life, the internal battery impedance may increase much more rapidly than would otherwise be expected. This parametric data measurement may be derived from two other parametric data measurements taken by the device--battery voltage and battery current.
Although the approaches described above for taking parametric data measurements during follow-up visits have been useful, they have been limited in the sense that they have not provided a convenient way to take such measurements between follow-up visits. In some circumstances, this could leave the medical practitioner without sufficient information to make an informed judgement about the current operating condition of the implantable cardiac stimulating device. For example, there are some situations where a partial lead fracture may only intermittently exhibit itself. The lead impedance may appear normal most of the time but occasionally, the partial lead fracture may cause a lead impedance fluctuation. Unless the medical practitioner happens to request a lead impedance measurement at the time of the lead impedance fluctuation (which means the patient happens to be visiting the medical practitioner at that time), there will be no indication of the partial lead fracture.
Also, there are times when the condition of the implantable cardiac stimulating device is better understood by evaluating trends in the parametric data measurements. For example, as mentioned above, the internal battery impedance rapidly increases near the end of its useful life. Therefore, by observing a recent trend of internal battery impedance measurements, one can quickly determine whether or not the implantable cardiac stimulating device should be replaced. But in order to evaluate trends in the data, it is necessary to take frequent measurements--and frequent measurements are impractical using the above-described approaches because follow-up visits are relatively rare. In addition, comparing current parametric data measurements to earlier measurements can be cumbersome when the above-described approaches are used, because the prior data must be retrieved from a source other than programmer/analyzer, such as a file cabinet or a separate computer system. And further, the above-described approaches do not conveniently allow for statistical calculations to be performed based on parametric data measurements taken over an extended period of time.
One cardiac stimulating device that can measure lead impedance is described in the commonly-assigned U.S. Pat. No. 4,899,750 (Ekwall), which is hereby incorporated by reference in its entirety. The Ekwall patent describes a device in which lead impedance is determined by measuring the change in the voltage on a discharge capacitor during the application of a pacing pulse. The system in Ekwall maintains a running average of the lead impedance measurements. If a current lead impedance measurement differs from the running average by more than a threshold amount for three consecutive pulses, then a counter is incremented. A medical practitioner can use the counter reading to evaluate whether the lead needs to be repaired or replaced. However, it is not possible for a medical practitioner using the Ekwall system to determine whether there is a trend in the lead impedance measurements that is a cause for concern--because the individual measurements are not retained.
It would therefore be desirable to be able to provide a medical practitioner with parametric data measurements that are taken at times other than during follow-up visits, so that the medical practitioner can better evaluate intermittent variations and trends in the measurements.